The present invention generally relates to the technical field of wireless communication, and more particularly, to an antenna system using multiple antenna elements and a weighting control technique for such antenna systems.
Adaptive array antennas (AAAs), which are in the picture of the technological field of wireless communication, use multiple antenna elements for transmitting and receiving radio signals. According to the ever-changing communication environment, the amplitude and the phase of a signal input to and output from each of the antenna elements are appropriately adjusted. The input signals to or the output signals from the respective antenna elements are weighted and synthesized to improve the signal-to-interference-plus-noise ratio (SINR). The adaptive array antenna technique is advantageous from the viewpoints of improving communication quality, reducing interfering waves, expanding the communicating range, and dealing with multipath fading. With a digital scheme of the adaptive array antenna technique, a weighting coefficient given to each antenna element is defined in a digital format. Such an adaptive array antenna system is disclosed in, for example, “Smart Antennas for Wireless Systems,” Jack H. Winters, IEEE Personal Communications, February 1998, pp. 23–27.
FIG. 1 illustrates a digital-based adaptive array antenna system. Each of N antenna elements 102 is furnished with an RF front end 104, an analog-to-digital converter 106, and a weighting unit 108. The weighting coefficients w1 through wN set at the respective weighting units 108 are determined by the weight controller 110.
With this arrangement, a digital signal is acquired separately from each of the antenna elements 102, and supplied to the weight controller 110. The weight controller 110 calculates the associated weighting coefficient accurately based on the digital signals. However, with this digital scheme, as many analog-to-digital converters 106 as the number of the antenna elements have to be prepared. This may be disadvantageous from the viewpoints of power saving and miniaturization of the system.
On the other hand, in some adaptive array antenna systems, weighting coefficients given to the antenna elements are analog-formatted. Such an analog-based adaptive array antenna system is disclosed in, for example, “Phased Arrays-Part I: Theory and Architectures”, Don Parker and David C. Zimmermann, IEEE Transactions on Microwave Theory and Techniques, Vol. 50, No. 3, March 2002, pp. 678–687, and “Phased Arrays-Part II: Implementations, Applications, and Future Trends”, Don Parker and David C. Zimmermann, IEEE Transactions on Microwave Theory and Techniques, Vol. 50, No. 3, March 2002, pp. 688–698.
With the analog scheme, the analog signal, which has been subjected to weighting and signal synthesis, is converted to a digital signal by the analog-to-digital converter 206 (FIG. 2). This arrangement is advantageous in power saving and size reduction. However, it takes time for this method to optimize all of N weighting coefficients w1 through wN. In addition, the weighting coefficient converges only to a local solution, without converging to the global optimum, due to less information being supplied to the weight controller 210. A global solution is the optimum weighting coefficient that is the maximum or the minimum in a given range of values. A local solution is the local optimum solution, which is the maximum or the minimum in a certain portion of the range, but is not necessarily the optimum in the entire range. It is desired to obtain the global optimum; however, with the conventional analog-based weighting control technique, the weighting coefficient may not converge to the global optimum, but only to the local solution, depending on how the initial value is set or on other conditions.